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Democritus never really "experimented." He believed everything was made up of atoms. He was one of the first people to consider that everything consisted of atoms.
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Dalton published a table of relative atomic weights in 1803. This table had 6 elements. He determined that the structure of compounds can be shown in whole number ratios.
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Thomson did experiments that showed cathode rays were not actually rays, but were negatively charged particles; electrons. He though that electrons were distributed randomly and made the plum pudding model.
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Rutherford did the gold-foil experiment. He found that the nucleus is positively charged, small, and dense. He also developed the Rutherford model of the atom.
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Bohr made the Bohr atomic model, which consists of a small, positively charged nucleus in the center, surrounded by circles of electrons. He discovered that outer layers could hold more electrons than inner layers.
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Louis De Broglie had a theory called the matter and wave particle duality theory, which set the basis of wave mechanics.
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Chadwick discovered that there was a particle lacking an electric charge. He also discovered that this neutron had about the same mass as a proton.